1. Field of the Invention
The present invention relates to an optical pickup for recording and/or reproducing a signal to/from an optical disk.
2. Description of the Background Art
A CD (Compact Disk) and the like having recessed pits arranged on a recording surface of an optical disk and allowing reproduction of digital audio signals and video signals by modulating dimension of the recessed pits dependent on the information to be recorded has been widely used. A CD-R (Recordable) having the same recording capacity as a CD and allowing recording only once has also been in the market. Further, a high density, reproduction-only DVD (Digital Video Disk) recording signals by the arrangement of pits similar to a CD and having the storage capacity of 4.7 Gbytes on one side and 9.4 Gbytes on both sides has also been put on the market. A DVD-RAM (Random Access Memory) has also been developed as a recordable optical disk having the same storage capacity as the DVD. Further, standard for a magneto-optical recording medium having the diameter of 12 cm and the storage capacity of 6 Gbytes has been established for practical application as a recordable optical disk.
When a signal is to be reproduced from an optical disk such as the CD, CD-R, DVD, DVD-RAM and the magneto-optical recording medium, a signal recording surface of the optical disk is irradiated with a laser beam, and change in intensity of the light reflected from the signal recording surface is detected. In this case, the necessary laser beam power is 0.2 to 1 mW. When a signal is to be recorded by the laser beam on the CD-R, DVD-RAM or the magneto-optical recording medium as a recordable optical disk, the power of several mW is necessary.
Referring to FIG. 1, a laser beam emitted from a semiconductor laser has an elliptical shape in a plane orthogonal to an optical axis (cross section), and the shorter diameter 130 is much different from the longer diameter 131. When a signal is to be recorded on an optical disk by using the laser beam, it is necessary to collect the laser beam by an objective lens. When the shorter diameter 143 of a laser beam 141 is shorter than the pupil diameter of an objective lens 140, while it is possible to well converge the laser beam along the direction of the longer diameter 142 of laser beam 141, it is impossible to sufficiently converge the beam along the direction 143 of the shorter diameter. As a result, the signal recording surface of the optical disk is irradiated with the laser beam having the elliptical shape, which leads to poor recording characteristic. When the shorter diameter 143 is made larger than the pupil diameter of objective lens 140 so as to sufficiently converge the laser in the shorter diameter 143 as well, the efficiency in using the laser beam lowers.
In view of the foregoing, an optical pickup has been adopted in which a laser beam emitted from a semiconductor laser 1 is collimated by a collimater lens 150, aspect ratio, that is, the ratio of the shorter diameter to the longer diameter of the beam is converted to such a value that ensures sufficient convergence of the laser beam both in the directions of the shorter and longer diameters by a beam shaping prism 151 or a cylindrical lens 155, and the beam enters an objective lens 152 and directed to an optical disk 6, as shown in FIGS. 3 and 4. When the optical disk 6 is irradiated with the laser beam by such a method, both the shorter and longer diameters of laser beam 153 are longer than the diameter of objective lens 153, and therefore the laser beam 153 can sufficiently be converged.
In the optical pickup shown in FIG. 3, however, the laser beam comes to have the optical axis changed after emission from the semiconductor laser 1 until being incident on optical disk 6, and therefore, it is difficult to align the optical axis. Further, the optical pickup shown in FIG. 4 has a number of optical components, and therefore it is difficult to fabricate a compact optical pickup.
An object of the present invention is to provide an optical pickup device capable of irradiating an optical disk with a laser beam having sufficient power to record a signal.
According to the present invention, the optical pickup device includes a laser, an objective lens and an optical member. The laser generates a laser beam having an elliptical cross section. The objective lens focuses the laser beam on an optical recording medium. The optical member is provided between the laser and the objective lens, and converts the laser beam from the laser to a beam having a desired aspect ratio by diffracting the laser beam outward along the direction of the shorter diameter of the beam. Therefore, it is possible for the objective lens to sufficiently converge the laser beam. As a result, efficiency in using the laser beam is improved, and the optical recording medium can be irradiated with the laser beam having sufficient power to record a signal.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.